In a sheet guide of a conventional printer, a head facing portion that faces a print head is rectilinear. Therefore, the print head and the head facing portion of the sheet guide have a substantially uniform space across the width between them. In this case, the leading end portion of a sheet that is printed by means of a print mechanism including the print head, especially its opposite ends, tend to abut against the sheet guide and cause jamming due to deformation of the sheet or the like as the leading end portion of the sheet passes through a gap between the sheet guide and a conveyor plate.
Referring now to the plan view of FIG. 5 and the sectional view of FIG. 6, there will be described a sheet guide of a conventional printer that can print continuous and cut sheets.
A carriage shaft 53 (indicated by two-dot chain line in FIG. 5) is stretched between left- and right-hand side frames 52 of a printer 51. A carriage 55 is supported on the carriage shaft 53 for reciprocation in the axial direction of the carriage shaft 53. A print head 56 is mounted on the carriage 55 in a manner such that it can reciprocate integrally with the carriage 55.
Conveyor plates 59 are arranged in front and at the back of the print head 56. A first sheet guide 61 and a second sheet guide 71 are arranged on these conveyor plates 59, individually.
The first sheet guide 61 is formed with a head facing portion 61a, which faces the print head 56 and extends parallel to the carriage shaft 53. Downward from the head facing portion 61a extends a sheet guide face 61b in the form of a slope, which serves to guide a sheet 60 (indicated by two-dot chain line in FIG. 5), printed by means of a print mechanism including the print head 56, to a conveyor path 62 that is defined between the undersurface of the first sheet guide 61 and the conveyor plates 59. Further, the second sheet guide 71, like the first sheet guide 61, is formed with a head facing portion 71a and a sheet guide face 71.
FIG. 5 shows a state in which the right-hand side portion of the leading end edge of the sheet 60, which is slightly skewed as it is fed to the printer, abuts against the head facing portion 71a of the second sheet guide 71. FIG. 6 is a view taken from the section side of the sheet 60 and showing the same state. If the right-hand side portion of the leading end of the sheet 60 is warped and lifted, as shown in FIG. 6, then the sheet 60, which is advancing forward, abuts against the second sheet guide 71, so that it cannot be guided by the sheet guide face 71b to get under the second sheet guide 71. Inevitably, therefore, the sheet 60 causes jamming in the printer.
If the sheet guide face 71b is located high in this case, the sheet 60 can be led to a position under the second sheet guide 71 even though the sheet 60 is warped substantially. However, if the height of the sheet guide face 71b is great, then the printer will be thick-profiled, which is not a favorable factor for the reduction in size and thickness of the printer.
In some printers, moreover, it is necessary to detect the presence of a sheet (e.g., to detect the leading end of the sheet or the like) that is conveyed in either of two opposite directions, forward and reverse, on the conveyor path. Preferably, the presence of the sheet should be determined by detecting the width-direction central portion of the sheet so that the sheet can be detected in a predetermined detecting position on the conveyor path without regard to its variation in width. To attain this, a sheet detecting mechanism is provided in a given position on the conveyor path through which the width-direction central portion of the sheet passes.
Many of conventional sheet detecting mechanisms comprise an optical sensor that is composed of a light emitting element and a light receiving element in a pair. One and the other of these elements are located individually at right angles to a conveyor path on either side thereof, facing a sheet detecting position on the conveyor path. The presence of a sheet is detected as an output signal from the light receiving element is turned on or off.
In the sheet detecting mechanism having the light emitting and receiving elements arranged separately on one and the other side of the conveyor path, however, two element parts are needed for detection, and a transit substrate for the arrangement of the other element must be provided on the other side of the conveyor path. Further, a harness for signal transmission must be connected between the transit substrate on the other side of the conveyor path and the one side of the conveyor path. Therefore, the harness must be made to extend long without interfering with the conveyor path, so that wiring is troublesome, and a connector for connection is need. Thus, the manufacturing costs increase correspondingly.
There is a method, moreover, in which a sheet is detected directly by means of a transmission-type photo-interrupter as a sheet detecting sensor. Although only one element part is need for detection, in this case, the width-direction central portion of the sheet cannot be detected, since the region to be detected is a width-direction end portion of the sheet.
Also known is a printer, such as a flat-bed printer, in which a sheet can be conveyed in either of two opposite directions, forward and reverse, on a conveyor path, for example. According to this version, the direction in which a continuous sheet is conveyed is opposite to the conveying direction for a cut sheet on the same conveyor path. In many printers, furthermore, a main control board, which is provided with control means for realizing a control system for controlling various functions (e.g., sheet feed, printing, carriage transfer, etc.), is located on the base plate of the printer body, while the sheet conveyor path is situated above the base plate of the printer body. It is advisable, therefore, to provide the main control board with sensors and other elements that require wire connection with the control means.